In the related art, an 802.16m system, as an evolution system of an 802.16e system, is a multi-carrier system based on Orthogonal Frequency Division Multiplex Access (OFDMA) for short) technology. The architecture of the multi-carrier system is as shown in FIG. 1. A single Media Access Control (MAC for short) entity controls a physical layer (PHY for short) comprising a plurality of channels, for example, the PHY includes a channel 1, a channel 2 . . . a channel N. Each channel may have a different bandwidth, e.g. the bandwidth of channel 1 is 5 MHz, the bandwidth of channel 2 is 10 MHz, and the bandwidth of channel N is 20 MHz. Moreover, the bandwidths of the above channels are continuous or discontinuous frequency bands.
At present, the carriers in a multi-carrier system (e.g. the 802.16m system) can be classified into two types, viz. a primary carrier and a secondary carrier, wherein the primary carrier refers to a carrier for interacting services and all PHY/MAC control information between a Base Station (BS for short) and a Mobile station (MS for short). In addition, the primary carrier is also used for controlling the operation of the MS, such as network access, etc. In a cell, each MS has only one primary carrier.
The secondary carrier is an additional carrier which is received on the primary carrier based on specific commands and rules of the BS for transmitting services for the MS. The secondary carrier may also comprise some control signaling supporting multi-carrier operations. In one cell, each MS may have one or more secondary carriers or no secondary carrier. The MS may obtain part of the information of the secondary carrier via the primary carrier, for instance, whether the secondary carrier is present, the position and configuration information of the secondary carrier, etc.
In the related art, the carriers in the multi-carrier system may have different configurations, e.g. a fully-configured carrier and a partially-configured carrier, wherein the fully-configured carrier refers to a carrier configured with all the control channels which include synchronization, broadcast, multicast, unicast control signaling channels and additionally, the information and parameter channels related to multi-carrier operations and other carriers. The partially-configured carrier refers to a carrier only comprising a basic control channel configuration used for supporting service interaction in the multi-carrier operations. In consideration of the application of the primary carrier and secondary carrier, the primary carrier has to be a fully-configured carrier, while the secondary carrier may be either a fully-configured carrier or a partially-configured carrier which depends on a usage and configuration model.
In a communication system, users within a base station can be classified into a user in the center of the cell and a user at the edge of the cell. As shown in FIG. 2, relative to base station 1, the users within the range of the circle 201 (a mobile station 1, a mobile station 2 . . . a mobile station 5) are users in the center of the cell, while the users within the range of a circle 202 (a mobile station 6, a mobile station 7, and a mobile station 8) are users at the edge of the cell. Of course, the circle 201 and the circle 202 are provided for illustratively distinguishing the two kinds of users. In a practical application, distinguishing between the users in the center of the cell and the users at the edge of the cell depends on actual conditions of the base station.
As for the user at the edge of the cell, since Quality of Service (QoS for short) is degraded due to channel fading, the MS will terminate the connection with the current serving base station and hand over to other adjacent base stations so as to obtain a better QoS. The above process is an inter-BS handover.
As for the user in the center of the cell of the 802.16m multi-carrier system, the carrier where a user is located might be overloaded with the increase of the services and the introduction of new services, while other carriers of a service BS might be lightly loaded. The carrier of the user can be handed over to other carriers of the serving BS to achieve a load balance for the system. In addition, different carriers of the discontinuous frequency band might experience various degrees of fading. Since the channel quality of the carrier where the user is located is degraded, the carrier of user can be handed over to other carriers of the serving BS with better channel quality so as to better meet the QoS demand of the user. The above process is called as an intra-BS carrier handover.
The 802.16e system is a single-carrier system with only the inter-BS handover present. The inter-BS handover process mainly includes a cell reselection, a handover decision and initiation, network re-access, context termination of a MS in a serving BS, etc. A certain handover interruption time delay is present in the inter-BS handover from the handover initiation to the successful re-access to a target BS.
The 802.16m system is a multi-carrier system with the inter-BS handover and the intra-BS carrier handover existing. When performing the intra-BS carrier handover, since the MSs are served by the same BS, handover interruption time delay will result in service discontinuity, such that seamless handover can not be achieved.